The embodiments described herein relate to an X-ray CT (Computed Tomography) apparatus for reconstructing a tilt image.
As one of the imaging methods using an X-ray CT apparatus, there has conventionally been known a method for performing a scan with a gantry being tilted by its tilt mechanism thereby to obtain a tilt image whose slice plane is being tilted at a predetermined tilt angle.
To scan a head portion of a subject for example, in such a manner that a tilt image suitable for diagnosis is obtained, a gantry is tilted to perform a scan in such a manner that a scan plane matches with a reference line such as an OM line (a straight line connecting the tail of the eye and the lughole) or the like by a projector of the gantry.
Providing the gantry with the tilt mechanism, however, has the following drawbacks. For example, extra space is required in a tilting direction to enable the tilting of the gantry, and space saving is not achieved. Further, for example, the width in a z direction of the gantry tends to increase with a detector's multirowing. Therefore, a scanning table and the gantry may interfere with each other, and the design is changed as needed. For example, the tilt mechanism of the gantry is complex in structure and is rigidly built for the enhancement of its rigidity in order to make it possible to stably support a heavy casing.
Accordingly, an X-ray CT apparatus which reconstructs a tilt image, based on projection data obtained by a non-tilt scan without providing the gantry with the tilt mechanism, is also considered.
Such an X-ray CT apparatus performs such imaging as shown in the following, for example.
FIG. 16 is a diagram for explaining a general scan range setting method at the acquisition of a tilt image by a non-tilt scan.
As shown in FIG. 16, an operator first sets a tilt image reconstruction range TR0 corresponding to a conventional tilt scan range on a scout image 92 of a subject 8 in its lateral direction. The X-ray CT apparatus sets scan conditions for scanning the space of the set tilt image reconstruction range TR0 and executes a non-tilt scan in response to instructions for the start of imaging. After the completion of the non-tilt scan, the X-ray CT apparatus reconstructs the tilt image in the tilt image reconstruction range, based on acquired projection data.
Here, the X-ray CT apparatus sets scan conditions so as to be able to scan all spaces included in the tilt image reconstruction range TR0, of a scannable space defined by a scan field of view SFOV in normal times. Namely, when viewed on the scout image 92 in the lateral direction, the X-ray CT apparatus sets a range SR0 in the z direction, of an area inclusive of the tilt image reconstruction range TR0 of an area WS corresponding to the scannable space, as a scan range for a non-tilt scan. This is done to acquire a tilt image of a region of interest KR0 even though a region of interest KR0 exists in any position in the tile image reconstruction range TR0.
In fact, however, the tilt image is not required over all the spaces in the tilt image reconstruction range TR0. The tilt image may be obtained only with respect to a spatial portion corresponding to part thereof, in which the region of interest KR0 exists. Therefore, in such a scan range setting method as described above, ranges F1 and F2 unnecessary for the image reconstruction are contained in the scan range SR0, and useless exposure to the subject increases.
In view of the foregoing, there has been desired an X-ray CT apparatus capable of obtaining a tilt image of a region of interest at low exposure without using a tilt mechanism of a gantry.